This invention is directed to a method of protecting galvanized steel from corrosion, i.e., white rust, by treating either zinc-coated metal or zinc-alloy-coated metal with effective amounts of a silica compound. More specifically, the invention is directed to a method of protecting galvanized metal from white rust by subjecting the galvanized coating, i.e., the zinc-coated metal, following the galvanizing process, to a treatment with silica at metal immersion temperatures at above about 125.degree. C. or higher to obtain a galvanized product resistant to corrosion known as white rust.
In the production of hot-dipped galvanized metals, such as sheet or tubing, the metal substrate is immersed or dipped into the molten metal for a time sufficient to obtain a uniform coating. Zinc coatings are generally characterized as having good adherence to metal substrate. Generally, the temperatures of the zinc bath range from about 435.degree. to 455.degree. C., or higher. However, temperatures in excess of about 480.degree. C. are avoided, since there is a tendency for the metal to oxidize at higher temperatures, and also requires additional time for the coating to solidify.
After the metal substrate has been galvanized, it is generally characterized as having a bright metal luster. Under storage conditions, however, the galvanized products are often subjected to humid conditions, whereby moisture causes the formation of what is known as "white rust" or "wet storage stain." Wet storage or white rust stains occur, for example, during shipment or storage of the galvanized product because of condensation which results from temperature change and humidity. The stain is characterized as a white or gray corrosion material which is primarily basic zinc carbonate, hydroxide, and zinc oxide.
This invention relates to the treatment of galvanized metals, e.g., zinc or zinc-containing alloys on steels, to inhibit wet storage staining or the formation of white rust. More particularly, it relates to the use of silica compounds immediately upon removing the galvanized metal from the molten zinc by subjecting the coated metal with a silica compound at metal immersion temperatures in excess of about 125.degree. C. or higher. The method of treating the galvanized surfaces in accordance with this invention provides protection against wet storage staining of zinc coatings and avoids, or substantially reduces and retards, the formation of white rust corrosion.
The presence of white rust generally impairs the appearance of the coated articles, and also prevents the adhesion of coating, such as paint and the like, to the metal surface. Thus, in view of the ever increasing demand for galvanized steel products, the problem of wet storage staining or white rust has become acute in the steel industry, and although many methods and compositions for treating galvanized steel and/or zinc surfaces to prevent white rust have been suggested, none have been found to be entirely satisfactory for one or more reasons.
Most common white rust inhibitors are based on the use of chromic acid or salts of chromic acid, in combination with various other compounds such as phosphates, silicates, and the like. Most chromates are toxic and present disposal problems in view of regulations promulgated by the Environmental Protection Agency.
For example, U.S. Pat. No. 3,687,738 discloses chromic acid compounds in an alcohol medium. Similarly, U.S. Pat. No. 3,907,608 suggests the use of pulverulent metal and chromium compounds in a liquid medium containing water and high boiling organic liquids. These coatings provide protection against rust upon exposing the surfaces to salt and the like. U.S. Pat. No. 4,659,394 discloses a process for the preparation of anticorrosive treated steel plate having zinc or aluminum-type coatings by use of a chromate film. U.S. Pat. No. 4,225,351 discloses a method of preventing corrosion of zinc-plated surfaces by treatment with a solution comprising sulfuric acid, hydrogen peroxide, and a silicate, with certain organic phosphorus compounds. U.S. Pat. No. 4,225,350 likewise discloses the resistance of zinc-plated surfaces to corrosion by treating the zinc surface with certain silicate-containing solutions and organic phosphorous compounds.
Thus, while a number of treatments have been developed to inhibit the formation of white rust, with varying degrees of success, there are a number of drawbacks, particularly when the treatments include the use of chromium compounds. Therefore, while the treatments have been useful for reducing white rust, they are considered objectionable due to the chromium salts, which impart an undesirable yellow color to the galvanized sheet and violate disposal regulations promulgated by the Environmental Protection Agency.